This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations are used in the description below:
3GPP third generation partnership project
ACK/NACK acknowledgement/negative acknowledgement
CSI cyclic shift index
DL downlink
DM RS demodulation reference symbols
e-NodeB/eNB node B (access node/base station) of an E-UTRAN system
E-UTRAN evolved UTRAN
FDD frequency division duplex
H-ARQ hybrid automatic repeat request
LTE long term evolution of 3GPP
MU-MIMO multi-user multiple input/multiple output
Node B base station or similar network access node, including e-NodeB
PBCH physical broadcast channel
PDCCH physical downlink control channel
PHICH physical H-ARQ indicator channel
PRB physical resource block
PUSCH physical uplink shared channel
PUCCH physical uplink control channel
TDD time division duplex
TTI transmission time interval
UE user equipment (e.g., mobile equipment/station)
UL uplink
UMTS universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
3GPP is standardizing the long-term evolution (LTE) of the radio-access technology which aims to achieve reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. Specifically, LTE employs the concept of the e-NodeB scheduling its own radio resources within the cell, which gives more flexibility to put available resources to use and also reduces latency in addressing uplink and downlink needs of the various user equipments in the cell. Its most flexible form is dynamic scheduling, where a single scheduling grant sent on a shared control channel (e.g., PDCCH) grants to one particular user equipment one particular amount of physical resources. This amount of physical resources is constructed of a number of uplink physical resource blocks. The Node B (or its surrogate in the case of relay stations) then must send an ACK or NACK as appropriate to the user equipment once that granted set of UL PRBs passes so the UE can know whether or not it must re-transmit its UL data. LTE sends the ACK/NACK on a special channel (PHICH). The ACK/NACK on the PHICH is made compatible with dynamic scheduling by mapping the UL resource granted to the UE to the particular PHICH where the ACK/NACK is to be. This FDD mode mapping from the PRB to the proper PHICH is readily extended to the TDD mode also, and LTE uses both modes.
So in the FDD mode of LTE, for each UL resource grant (signalled on the DL PDCCH) there will be an associated H-ARQ feedback channel for positive (ACK) or negative (NACK) acknowledgements of data that the UE sends on that granted UL resource. In the current understanding in 3GPP, there will be a delay between the time of the UL grant (on the PDCCH) to the time where the UE will actually transmit its uplink data on the granted UL resources, and further another delay to the time where the eNodeB should send the ACK/NACK on the PHICH responding whether or not the eNodeB received the UE's UL data. The current assumption is that the scheduling delay will be 3 ms (plus the delay of the actual signalling on the PDCCH), and that the eNodeB processing time will also be 3 ms. Assuming one TTI=1 msec and indexing TTIs from zero, this means that the timing relation for a single H-ARQ process or channel will be that the UL allocation grant on the PDCCH is at TTI#0; the UL data transmission will be no earlier than TTI#4, and the ACK/NACK on the PHICH will be no earlier than TTI#8. Instead of the ACK/NACK the eNodeB can send a dynamic scheduling (on the PDCCH), which is termed adaptive H-ARQ but still subject to the same minimum time delay.
Related to the H-ARQ operation, there is also the need to convey the reception status to the UE that originally transmitted the data packet to the e-Node B. For this there are several options. For the case that adaptive H-ARQ is used for the uplink, retransmissions are dynamically scheduled. For the case of continuous traffic, a packet on the granted UL resource is acknowledged by the e-Node allocating resources for the UE to send the next data packet. The case where the UE has no further/new data to send is where the PHICH becomes relevant. The PHICH is a per-UE dedicated channel which is temporarily assigned to the UE which transmitted the data packet, and as above is tied to the allocated uplink physical resources so that the proper data packet can be identified with a particular ACK/NACK on the PHICH. See U.S. Provisional Patent Application 61/010,354, filed Jan. 7, 2008 and entitled “Method, Apparatus and Computer Program to Map a Downlink Resource to a Related Uplink Transmission” (now U.S. patent application Ser. No. 12/349,683, filed Jan. 7, 2009). See also: R1-080301, PHICH and mapping to PHICH Groups, 3GPP TSG RAN WG1 MEETING #51BIS, Sevilla, Spain, Jan. 14-18, 2008 by Nokia and Nokia Siemens Networks.
For the case where non-adaptive H-ARQ is used for uplink (meaning that uplink retransmissions are performed on the same physical resources), the UE only needs an indication of whether or not it should do a retransmission in the uplink. This is handled through PHICH signalling. While not yet settled in LTE, it is expected that the UE would probably be assigned a PHICH resource (or PHICH channel) through its allocated physical resources for the uplink transmission. For the case where multi-user MIMO is used, there is the potential for a collision of the assigned PHICH channels. MU-MIMO is a special case of dynamic allocations that might be used where the same uplink transmission resources are allocated to two or more users at the same time.
The UEs are divided to one or more groups and for each UE group a PHICH group is assigned, a PHICH group consisting of physical resources that can at maximum carry 8 ACKs/NACKs (in the case where a short cyclic prefix is used; for a long cyclic prefix the number might be less). The UE knows the ACK/NACK resources inside the PHICH group from the CSI of the DM RS, which is signaled to the UE in its UL grant for the corresponding UL transmission. This CSI is 3-bits and with these bits the exact ACK/NACK inside the PHICH group can be identified (or in another group, see 3GPP TS 36.213).
This previous approach is also applied for the MU-MIMO case. In the MU-MIMO case, two users at different channel conditions are assigned to the same physical (time/frequency) resources and their transmissions can be decoded in the e-NodeB due to those different channel conditions (e.g. different physical locations). The ‘same’ resources may be overlapping rather than wholly identical. In order to differentiate multiplexed UEs sharing the same physical resource at the receiver side, there should be low enough cross-correlation between the reference signals. This can be arranged by allocating different cyclic shift of the same DM RS for different MU-MIMO terminals. In this case, signaling of the cyclic shift can be done using dynamic CSI included in the UL grant (3 bits).
Currently, the suggestions known to the inventors for mapping between the granted UL resources and the PHICH that relates to the data on those granted UL resources has been focused on ensuring that there is a consistent configuration of the physical channels themselves. In the inventors' view this ignores a key aspect that would lead to the most efficient implementation when considering the entire radio frequency picture. The CSI is used to find the proper PHICH, but the CSI itself affects the cross-correlation properties between the UEs' transmissions. What is needed in the art is a comprehensive solution to map a UL resource to a DL resource on which the ACK/NACK for that UL resource is sent while taking into account the effects that different CSI has on the UEs.